Social stress is arguably the predominant form of stress encountered by mammals, and in humans this type of stress contributes to a variety of diseases and psychopathologies. Most research on stress effects do not use social stressors, however. Instead, many animal models of human stress-related disorders use stressors such as intermittent foot shock that offer the benefit of being highly controllable but that may bear little resemblance to stressors encountered in the lives of humans or non-humans. Animal models using a social context closer to that which individuals might experience in their natural environment (so-called ethological models) are essential to a better understanding of the neural mechanisms underlying social behavior, in general, and experience-dependent behavioral plasticity, in particular. We propose that conditioned defeat in hamsters is a unique, ethologically relevant model of stress-induced behavioral plasticity wherein a single, brief exposure to a social stressor reliably induces profound and long-lasting changes in social behavior. We also emphasize that studying models such as conditioned defeat is critical to an improved understanding of stress-related psychopathologies in humans and to the development of better treatment options for these disorders. We have demonstrated that the basolateral nucleus of the amygdala is critical to the acquisition and expression of conditioned defeat in hamsters, but we also have evidence that other brain areas are involved in the production of this behavioral change. The specific aims of this project will 1) define the broader neural Circuitry mediating conditioned defeat. This is important because it will allow us to concurrently study multiple brain systems that are usually examined in isolation from one another and will ultimately help us to understand how the brain "selects" which of a broad repertoire of behaviors is elicited in a particular situation. We will also 2) determine where in the neural circuit the critical plasticity occurs to mediate conditioned defeat and 3) describe some of the cellular/molecular changes that underlie social stress-induced changes in behavior. Social conflict is a major form of stress for humans, and exposure to this type of stress contributes to a variety of diseases and psychopathologies including depression and anxiety disorders. Understanding of the neural events that underlie conditioned defeat, a social stress-induced behavioral change, will improve our understanding of how stress changes the brain and subsequent behavior and should lead to the development of better treatment options for individuals with mood and anxiety disorders.